The present invention relates generally to a moisture application system for adding moisture to a web of paper, paperboard or the like for curl control, or to improve the performance of the paper for a particular end use application, or for improved finishing during a calendering operation. In its most practical sense, the present invention relates to a method and apparatus for the finishing of paper and paperboard, and more particularly to an improvement to a calender device normally used to provide enhanced smoothness and gloss to such products.
It is common practice in the paper industry to add moisture to paper webs during the manufacturing process as shown for example in U.S. Pat. No. 3,948,721. It is also known, for example, as shown in U.S. Pat. No. 2,130,530, to add moisture to a paper web in the finishing step at the machine calender during the papermaking process. This moisture application may take many different forms, but it is most commonly carried out by waterbox calendering. However, the use of a waterbox calender to impart a smooth surface to bleached paperboard, or a similar substrate, has an inherent problem, namely, a lack of control of the amount of liquid picked up by the substrate in the calender nip. During conventional waterbox calendering, the amount of liquid supplied to the calender nip is such that the nip is flooded. Thus, the quantity of liquid picked up is determined by the diameter of the calender rolls, operating speeds, nip pressures, and substrate characteristics (e.g., thickness, sizing level and roughness). Accordingly, the application of moisture to a paper substrate using a waterbox generally results in the transfer of an amount of liquid far in excess of what is required to achieve the desired smoothness. The excess liquid weakens the substrate resulting in web breaks, and tends to establish a lower basis weight limit for production using a waterbox. For some applications, penetration of the excess water into the substrate also results in an undesirable reduction in the caliper or thickness of the web. Thus there is a need to provide a means for applying moisture to a paper web during the calendering process that is independent of the speed of the calender. There is also a need to provide a means that introduces only as much moisture as is required to achieve the desired finished properties of the web without unduly influencing the thickness of the web.
Other methods for adding moisture to a paper web at the machine calender include the application of steam or water sprays to the web. The application of steam onto a web to increase its water content is possible, but it requires that the web be cooled for efficient condensation of the steam into the form of water droplets. An article entitled "Practical Aspects of Calender Steam Showers", by R. N. Vyse and David J. Savly, October 1998 TAPPI Journal, pp. 87-90, discloses the treatment of a paper web with steam before calendering. Spraying liquid directly onto a web is another method for increasing moisture content at the machine calender. However, water spray systems generally have limitations, primarily due to a lack of uniformity of application, and the production of wet streaks caused by an overlap of sprays from adjacent nozzles, which results in nonuniform smoothness and caliper profiles. These problems have been overcome to some extent with the application of moisture directly onto one of the rolls of a calender using a brush-spray device just prior to the point where the roll contacts the web, substantially as disclosed in U.S. Pat. No. 5,607,553, assigned to the present assignee herein. Likewise, moisture may also be added to a paper web at the machine calender by applying a metered film of liquid directly onto a calender roll as shown, for example, in U.S. Pat. No. 5,522,312, also assigned to the present assignee herein. The latter patent discloses an apparatus for adding small quantities of moisture to a paper web by mounting a metering element on the waterbox which meters excess liquid off the calender roll prior to the liquid transfer nip. While this apparatus and method has its merits, it has not been found to be completely successful in practice. Accordingly, while the use of a waterbox is generally agreed to be the preferred method for adding moisture to a paper web at a machine calender, the problems inherent with conventional waterbox calendering have yet to be solved. Thus it may be seen that a solution to these problems is desirable, and the solution proposed by the method and apparatus described herein represents a novel effort toward that end.